ABSTRACT Alcohol abuse is a significant global health problem. Chronic alcohol drinking can induce alcoholic liver disease, which ranges from steatosis (fatty liver) to steatohepatitis, fibrosis and cirrhosis. About 90% of heavy alcohol drinkers develop alcoholic fatty livers (AFL), an onset of alcoholic liver disease. Recently, we found that CYP2A5 (CYP2A6 in humans) is induced by chronic ethanol feeding and CYP2A6 is elevated in alcoholic patients. CYP2A5/6 is a major nicotine metabolic enzyme. Alcohol and tobacco are frequently co-abused and tobacco smoke can increase AFL. We found that nicotine, a major addictive forming alkaline in tobacco smoke, can enhance AFL and hypertriglyceridemia (HTG), which was observed in WT mice but not in cyp2a5-/- mice. PPAR?-regulated FGF21 is a novel metabolic regulator highly expressed in liver. We observed a constitutive elevation of hepatic PPAR?-FGF21 in cyp2a5-/- mice. Ethanol-induced HTG observed in ppar?-/- mice with an extremely low serum FGF21 can be blocked by the treatment of rFGF21. Ethanol/nicotine-induced AFL was more pronounced in liver-specific FGF21 KO mice. These results suggest that FGF21 may play an important role in FGF21 modulates cellular activity through FGF receptor 1 (FR1), which is mainly expressed in adipose tissues and to a much lesser extent in liver. We didn?t observe any difference in AFL and HTG between liver- specific FR1 KO mice and their WT control mice, suggesting that in our model the more important might be adipose FR1 but not liver FR1. Liver FGF21 can be released into blood to act in an endocrine manner. FGF21 can stimulate adipocytes to secret adiponectin, which in turn acts on the liver to ameliorate AFL. In AIM 1, we will reintroduce CYP2A5 back to cyp2a5-/- mice via AAV8 to validate the essential role of CYP2A5 in the nicotine-enhanced AFL and HTG and apply cotinine and antioxidant to examine the role of CYP2A5-produced nicotine metabolites and oxidative stress in the nicotine-enhanced AFL and HTG. In AIM 2, we will examine the role of a PPAR?-FGF21 axis in the nicotine-enhanced AFL and HTG by applying ppar?-/-/cyp2a5-/- mice, liver- specific FGF21 knockout mice and PPAR? specific agonist WY-14,643. In AIM 3, cell-cell interaction in cell co- culture system of adipocytes and hepatocytes will be examined to reflect organ-organ interaction between adipose tissue and liver. Adiponectin knockout mice will be treated with rFGF21 to evaluate if liver FGF21 exerts its action in adipose tissue through adiponectin i.e. the PPAR?-FGF21-adiponectin to regulate nicotine- enhanced AFL and HTG. At last, adiponectin and CYP2A5 double knockout mice will be applied to investigate the role of adiponectin in the observation that nicotine-enhanced AFL and HTG was observed in WT mice but not in cyp2a5-/- mice.